1. Field of the Invention
The present invention relates to a wireless communication system which relies on multi-valued modulation for transmitting an error correcting code and information of one bit or more per symbol, and a demodulation method and a data rate control method associated with the wireless communication system for improving the reception quality when the gain, interference, and/or noise power vary on a channel.
2. Description of the Related Art
The multi-valued modulation has been known for transmitting information of a plurality of bits per symbol in order to increase the transmission amount per unit time in a wireless communication system.
The multi-valued modulation exhibits a higher maximum throughput on a channel of high quality as the number of bits per symbol increases, but experiences a largely reduced throughput, if the channel is degraded in quality, as the number of bits per symbol increases. Thus, adaptive modulation has been proposed for switching the modulation level in accordance with the channel quality in order to ensure stable communications. The adaptive modulation is described, for example, in an article entitled “Performance of Modulation Level Controlled Adaptive Modulation Systems” (Transactions of the Institute of Electronics, Information and Communication Engineers B-II, Vol. J78-B-II, No. 6, pp. 435–555, June 1995).
Normal transmission data is processed in the following manner. In an originating communication device, a channel encoder first encodes data to be transmitted. While wireless communications often employ convolution encoding and turbo encoding as an encoding method, the encoding method need not be limited to those, but any other encoding method may also be applied to wireless communications. Encoded codewords are applied to a QAM (Quadrature Amplitude Modulation) adaptive modulator for modulation, and a resulting baseband signal after the modulation is converted to a signal in a radio frequency band by a RF module for transmission. On the other hand, a receiving wireless communication device receives an RF signal through an antenna, and converts the received RF signal to a signal in a base band in a RF module. The baseband signal is first demodulated by a QAM demodulator which inputs the resulting signal demodulated thereby (likelihood) to a channel decoder which decodes the demodulated signal.
In the adaptive modulation scheme, the modulation level in the originating communication device must be matched with the demodulation level in the receiving communication device. Specifically, the modulation level m in the QAM adaptive modulator of the originating communication device is determined in the following manner. First, the situation on a channel immediately before a received signal is demodulated in the receiving communication device (or before a current time) is measured using an S/N measuring device or the like. Next, an S/N information generator generates a signal for transmitting the result of the measurement from the receiving communication device to the originating transmission device. Then, a modulator modulates the generated information for transmission. In the originating communication device, the received signal is demodulated in a demodulator to acquire the channel situation such as S/N information. The channel situation is determined from the acquired S/N information and the like, and the modulation is performed at a high modulation level if a good situation is found on the channel, whereas the modulation is performed at a low modulation level, if a bad situation is found on the channel. A series of these control operations form a feedback system.
With the conventional adaptive modulation described above which switches the modulation level in accordance with the varying situation on the channel, the receiver needs to know a scheme used by the transmitter in the modulation, so that the transmitter must communicate information on the modulation scheme to the receiver.
Further, when an error occurs in the information on the modulation scheme, the receiver will incorrectly demodulate signals, resulting in a failure of data communication, so that the information on the modulation scheme must be communicated with high accuracy.
In addition, the requirement for correctly matching the modulation scheme between the transmitter and receiver as mentioned above causes a time delay from the measurement of the channel quality to the switching of the modulation scheme, so that the receiver cannot appropriately switch the modulation scheme in response to a sudden change on the channel, thereby failing to sufficiently utilize the channel for communications.